1. Field of the Invention
This invention relates to a strobe light-emission control apparatus.
2. Description of the Related Art
An autodimming strobe emits strobe light toward a subject and receives reflected light from the subject by a photoreceptor sensor. The signal that is output from the sensor is integrated by an integrating circuit and the light emission from the strobe is halted when the integrated value attains a predetermined threshold value.
When a light source (extraneous light) falls within a range in which it is sensible by the photoreceptor sensor, however, not only reflected strobe light but also light emitted from the light source is received by the photoreceptor sensor. A signal obtained based upon the light emitted from the light source also is integrated by the integrating circuit. As a consequence, the integrated value attains the predetermined threshold value earlier than the time at which the strobe light emission should be terminated and, hence, there are instances where the image obtained is too dark.
Further, when the subject is a short distance away, it is required that the autodimming strobe control the light emission comparatively accurately so as to reduce the total amount of light that illuminates the subject. However, since it is comparatively difficult to accurately control the light emission of a strobe flash unit, often accurate control of the total amount of light that illuminates the subject is difficult to achieve.
Accordingly, an object of the present invention is to exclude the effects of extraneous light in control of a strobe light emission.
Another object of the present invention is to control, in comparatively accurate fashion, the total amount of light that illuminates a subject.
Accordingly to a first aspect of the present invention, the foregoing objects are attained providing a strobe light-emission control apparatus comprising: a photoreceptor for outputting a signal that conforms to amount of received incident light; a strobe light-emission control circuit for controlling a strobe flash device in such a manner that a subject is illuminated with strobe light; a subtracting circuit for subtracting, from a first signal that is output from the photoreceptor at emission of strobe light from the strobe flash device, a second signal that was being output from the photoreceptor during non-emission of strobe light from the strobe flash device; an integrating circuit for integrating a signal obtained by subtraction by the subtracting circuit; and a strobe light-emission halt control circuit for controlling the strobe flash device so as to halt emission of strobe light based upon amount of integration by the integrating circuit.
A control method suited to the strobe light-emission control apparatus of the present invention may also be provided. Specifically, there is provided a method of controlling a strobe light-emission control apparatus having a photoreceptor for outputting a signal that conforms to amount of received incident light, and a strobe light-emission control circuit for controlling a strobe flash device in such a manner that a subject is illuminated with strobe light, the method comprising the steps of: subtracting, from a first signal that is output from the photoreceptor at emission of strobe light from the strobe flash device, a second signal that was being output from the photoreceptor during non-emission of strobe light from the strobe flash device; integrating a signal obtained by subtraction; and controlling the strobe flash device so as to halt emission of strobe light based upon amount of integration.
In accordance with the first aspect of the present invention, a signal conforming to amount of incident light is output from the photoreceptor. The second signal is output from the photoreceptor when strobe light is not being emitted by the strobe light-emission control circuit. (If necessary, a detection circuit for detecting the second signal would be provided.) If extraneous light falls within the light-receiving range of the photoreceptor, the second signal indicates the amount of this extraneous light. The first signal is output from the photoreceptor when strobe light is being emitted by the strobe light-emission control circuit. The first signal indicates the total of extraneous light and strobe light that has been reflected from the subject.
The second signal is subtracted from the first signal by the subtracting circuit, and the signal that results from the subtraction operation is integrated by the integrating circuit. When the amount of integration by the integrating circuit attains a predetermined threshold value, the emission of strobe light is halted.
The subtracting circuit integrates the signal from which extraneous light has been excluded, i.e., the signal indicating the reflected strobe light. Control for halting the emission of light by the strobe flash device can be performed comparatively accurately.
Preferably, the subtracting circuit subtracts the second signal, which was being output from the photoreceptor at non-emission of strobe light immediately prior to emission of the strobe light, from the first signal for a period of time from emission of strobe light that is based upon control by the strobe light-emission control circuit to halting of emission of strobe light that is based upon control by the strobe light-emission halt control circuit. Thus, the effects of extraneous light can be excluded during emission of the strobe light.
Accordingly to a second aspect of the present invention, the foregoing objects are attained providing a strobe light-emission control apparatus comprising: a photoreceptor for outputting a photoreception signal that conforms to amount of received incident light; a first strobe light-emission control circuit for controlling a strobe flash device so as to illuminate a subject with strobe light and halt emission of light in response to a strobe light-emission halt signal applied thereto; an integrating circuit, which is reset with start of light emission by the strobe flash device and is reset in response to a reset signal applied thereto, for integrating the photoreception signal output from the photoreceptor; a comparison circuit for comparing an integrated value from the integrating circuit and a first reference signal and outputting a detection signal in response to the first reference value being surpassed by the integrated value; and a first control circuit for outputting the reset signal to the integrating circuit in response to output of the detection signal, which is output from the comparison circuit, prior to elapse of a first predetermined time from start of light emission by the strobe flash device, and outputting the strobe light-emission halt signal to the first strobe light-emission control circuit in response to output of the detection signal after elapse of the first predetermined time from start of light emission by the strobe flash device.
A method of controlling the strobe light-emission control apparatus of the second aspect of the present invention may also be provided. Specifically, there is provided a method of controlling a strobe light-emission control apparatus having a photoreceptor for outputting a photoreception signal that conforms to amount of received incident light, a first strobe light-emission control circuit for controlling a strobe flash device so as to illuminate a subject with strobe light and halt emission of light in response to a strobe light-emission halt signal applied thereto, and an integrating circuit, which is reset with start of light emission by the strobe flash device and is reset in response to a reset signal applied thereto, for integrating the photoreception signal output from the photoreceptor; the method comprising the steps of: comparing an integrated value from the integrating circuit and a first reference signal and outputting a detection signal in response to the first reference value being surpassed by the integrated value; and outputting the reset signal to the integrating circuit in response to output of the detection signal prior to elapse of a first predetermined time from start of light emission by the strobe flash device, and outputting the strobe light-emission halt signal to the first strobe light-emission control circuit in response to output of the detection signal after elapse of the first predetermined time from start of light emission by the strobe flash device.
In accordance with the second aspect of the present invention, a subject is illuminated with strobe light from a strobe flash device. Reflected strobe light from the subject is received by the photoreceptor, which outputs a photoreception signal. The strobe light-emission control apparatus is provided with an integrating circuit that is reset with start of light emission by the strobe flash device. The photoreception signal is integrated by this integrating circuit. The integrated value from the integrating circuit and a first reference value are compared by the comparison circuit, which outputs a detection signal when the integrated value exceeds the first reference value. In a case where the detection signal is output prior to the elapse of a first predetermined time from start of light emission by the strobe flash device, a reset signal is output to the integrating circuit to reset the same. Thus the integrating operation of the integrating circuit is performed from the beginning. In a case where the detection signal is output after elapse of the first predetermined time from the start of the light emission by the strobe flash device, a strobe light-emission halt signal is output to the strobe light-emission control circuit, whereby the strobe light emission of the strobe flash device is halted.
The characteristic of the light that exits from the strobe flash device rises sharply immediately after the start of the light emission and then declines gradually. In a case where control such as for halting the strobe light emission is performed immediately after the start of the light emission, therefore, it is necessary to perform control comparatively accurately. The second aspect of the present invention is such that in a case where the detection signal is detected within the first predetermined time from the start of the strobe light emission, the integrating circuit is reset, the integrating circuit integrates the photoreception signal obtained at such time that the light exiting the strobe flash device has become small and control for halting the strobe light emission based upon the integrated value is carried out again. The total amount of light that illuminates the subject can be controlled comparatively correctly even if control for halting the strobe light emission is not accurate. It goes without saying that if control for halting the strobe light emission is carried out again in this strobe light-emission control apparatus, the subject is imaged in sync with this re-execution of control and the image data obtained by such imaging is recorded on a recording medium. Further, if control for halting the strobe light emission is not carried out again, image data obtained by imaging the subject in sync with the start of the light emission from the strobe flash unit would be recorded on the recording medium.
The strobe light-emission control apparatus described above may be applied to an electronic digital camera having a solid-state electronic image sensing device for sensing the image of a subject and outputting a video signal representing the image of the subject, and a diaphragm placed in front of the photoreceptor surface of the solid-state electronic image sensing device. In this case, stored electric charge in the solid-state electronic image sensing device would be reset in response to output of the reset signal from the first control circuit. The apparatus further comprises a second control circuit for controlling the diaphragm so as to reduce the aperture, comparing the integrated value from the integrating circuit and a second reference value that is greater than the first reference value, and controlling the comparison circuit so as to output the detection signal in response to the second reference value being surpassed by the integrated value.
Since the diaphragm aperture is made small, the amount of light per unit time that impinges upon the photoreceptor surface of the solid-state electronic image sensing device is reduced. Image data representing an image having a comparatively appropriate amount of exposure is obtained even if timing of control for halting the strobe light emission is not accurate.
The second control circuit is so adapted that control of the diaphragm and control of the comparison circuit is performed in response to output of the detection signal from the comparison circuit to the strobe flash device prior to elapse of a second predetermined time, which is shorter than the first predetermined time, from the start of strobe light emission.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.